The Boeing 767, of low cost carrier Excel Airways, was on a flight from London's Gatwick airport to the Red Sea resort of Hurghada in Egypt.
The pilot asked Italian authorities for permission to land at the nearest airport, and the Italian air force scrambled an F-16 fighter to escort the Boeing into Brindisi, according to Italian news agencies.
The plane made an emergency landing at Brindisi airport and the 269 passengers were immediately evacuated along with the nine crew.
A search by police and firemen revealed no bomb and the plane was expected to continue to its destination.
A statement on the company website said the message, written on the back of a sick bag, stated that there was a bomb on the aircraft, and was passed among passengers before being handed to a member of cabin crew.
"The captain was advised and, as a precautionary measure, made the decision to divert into Brindisi."
Excel, which specialises in low cost holiday charter flights, added: "All passengers and bags for this flight had been security checked in the normal manner, but with heightened security the captain believed it was prudent to divert so that checks could be carried out."